Semiconductor wafers and other substrates may be provided with indicia that indicate, among other things, wafer number and manufacturer. The indicia may be, for example, 3/4 to 5/8 inches in length, and are typically constituted as a matrix of laser-etched pits.
The difficulties in providing machine reading of such laser-etched indicia on semiconductor wafers arise from the fact that the characters themselves are the same color as their wafer substrates and show little relief, from the fact that the wafer surface is polished, so that it reflects thereof the light used to read the characters, and from the fact that the surface of the wafer may undergo several coating processes which cover the characters and reduce their size and relief.
The heretofore known optical character recognition systems have utilized the small surface irregularities introduced by the indicia to reflect light to a camera. But because the dots are so smooth, they disperse light only through a small angle, and in some instances, extremely small angles. As a result, the source of the light must be at a very small angle from the line of sight and it must be relatively intense. This places the source in the camera's field of view and the result is, in many instances, that the camera is blinded. While prior art techniques which use beam splitters to direct light along the line of sight have been utilized, these techniques have not been robust enough to illuminate small characters with many overcoat layers. High intensity fiber optic light sources represent another prior art technique that, while providing very intense light which functions at greater angles and illuminates even the most difficult characters, is subject to the considerable disadvantage that they are expensive, large, and generate unwanted heat and vibration.